In a general communication system, in order to enable a receiver to correct an error which occurs in a channel, information transmitted by a transmitter is transmitted after being coded using a forward error correction (FEC) code. The receiver demodulates the received signal, decodes the FEC code, and restores transmitted information. In such a decoding process, the error of the received signal which occurs in the channel is corrected. Although various kinds of FEC codes may be used, for example, a turbo encoder will be described in the following description.
The turbo encoder is constituted by a recursive systematic convolution encoder and an interleaver. When the turbo encoder is actually implemented, an interleaver for facilitating parallel decoding, such as quadratic polynomial permutation (QPP) interleaver, may be used. The QPP interleaver has excellent performance in only a specific data block size. The performance of the turbo encoder is improved as the size of the data block is increased. In an actual, communication system, for convenience of actual implementation, a data block having a predetermined size or more is divided into several small data blocks and encoding is performed.
Each of the divided small data blocks is called a code block. The code blocks generally have the same size. However, due to the limitation of the size of the QPP interleaver, one of the several code blocks may have a different size. In order to reduce the influence of a burst error generated upon a transmission using a wireless channel after performing an FEC coding process in the unit of code blocks having a predetermined interleaver size, interleaving may be performed. In addition, the interleaved information is transmitted after being mapped to an actual radio resource.
Since the amount of radio resource used for an actual transmission is constant, rate matching is performed with respect to the encoded code blocks. Generally, the rate matching is performed by puncturing or repetition. The rate matching may be performed in the unit of encoded code blocks like the wideband code division multiplexing access (WCDMA) of the 3rd generation partnership project (3GPP). Alternatively, the rate matching may be separately performed with respect to a systematic part and a parity part of each of the encoded code blocks. This is shown in FIG. 1.
FIG. 1 is a schematic view showing a process of encoding a code block, performing rate matching with respect to the encoded code block, and transmitting the code block.
In the example of FIG. 1, each code block may be divided into a systematic bit stream and two parity bit streams by turbo encoding. These bit streams are subjected to sub-block interleaving and are then subjected to circular buffer rate matching. FIG. 1 shows a method of dividing the code block into the systematic part and the parity part and performing sub-block interleaving. The interleaved information is transmitted in the unit of information having a predetermined size.
Meanwhile, a hybrid automatic repeat request (HARQ) technology is obtained by combining channel coding and an ARQ technology, and improves decoding performance by retransmitting a data block in which an error occurs and combining the retransmitted data block and a previously transmitted data block. The HARQ scheme may be classified according to regulations of a time point when a retransmission is performed, and may be classified into an asynchronous HARQ scheme in which the time point when the retransmission is performed is variable, and a synchronous HARQ scheme in which the time point when the retransmission is performed is fixed. The HARQ scheme may be classified into a chase combining (CC) scheme and an incremental redundancy (IR) scheme according to types of a redundancy version (RV) used for the retransmission. In the CC scheme, a gain of a signal-to-noise ratio (SNR) is obtained by transmitting the same data block as a previous transmission. In contrast, in the IR scheme, a coding gain is obtained by transmitting data including a redundancy version different from that of a previous transmission.
If the HARQ scheme is applied in a system using circular buffer rate matching shown in FIG. 1, the RVs specify transmission start points of data blocks in the circular buffer so as to implement the IR scheme. That is, the start points should be defined in the circular buffer by the number of RVs.
Meanwhile, if data is transmitted using an M-QAM (M>4) modulation scheme, the constellation of a QAM symbol is composed of log2M bits. At this time, error performance is changed according to the bit locations in the symbol. That is, the error performance of a specific bit location may be more excellent than that of another specific bit location. In consideration of such a characteristic, if the locations of bits transmitted in a previous transmission are changed upon a retransmission of the HARQ system, a diversity gain is obtained and thus the error performance can be improved. This scheme is called a constellation rearrangement scheme. Although a constellation rearrangement gain can be mainly in the CC scheme, if a coding rate is low in the HARQ system using the IR scheme, many portions of the RVs overlap. Thus, even in this case, a gain can be obtained.